It is a common task on construction sites to use devices for detecting underground structures before or while earth-moving. Such structures are often occurring in form of services for supplying electricity, gas, fuel, water, or communication data, etc. by underground structures. Although the location of most of these services is or at least should be already known from a surveyor's plan of the site, their locations can have uncertainties or there could be additional services that are not mentioned therein. Often underground services are also simply overlooked or wrongly assessed by the operator of an earth moving machine during work.
Avoidance of damage to underground structures while digging in a trench or in areas being excavated is an important task. As damage to a service can cause serious impact and costs, additional measurements are taken in order to be able to detect the proximity or preferably the exact depth of such services on the site before or while excavating. Therein, it is not only of interest to determine the path, which the buried service is following, but also to determine the depth at which the service is buried, or in other words to determine the distance from the detection device to the service. The distance from the device to the service will further also be referred to as depth, as a common term used for underground wirings, conduits or pipes. Devices for this purpose are known as Cable Detection Tools or Cable Avoidance Tools—also called CAT. An embodiment of such a device is for example described in EP 2 362 241. Such a detection device is mostly embodied movable, which means it can be designed and built as a handheld device to be carried around by a worker. In special movable embodiment of the detection device, it can for example also be mounted at a bucket of an excavator and move with the bucket. In view of this, the detection device is preferably embodied lightweight and small-sized.
One way to locate underground services is to detect electromagnetic fields sent out by the service itself. To do this, the service requires having a naturally occurring electrical signal which emits an electromagnetic field that is detectable above the ground, such as e.g. a live power supply line. To detect other types of services as well, for example a wiring system of switched off street lights, unused or low-voltage communication cables, gas- or water-pipes, additional methods are known. In U.S. Pat. No. 4,438,401 metallic services without naturally occurring signals are directly connected to a signal-generator. In this way, an electrical signal can be coupled to the service, and therefore it is possible to detect it by its electromagnetic field. U.S. Pat. No. 5,194,812 shows a solution for detecting hollow pipes like gas or water pipes by introducing a conductor or sonde into them—or by laying a conductor next to the service—that will function as a transmitter for the field to be detected. In EP 9 166 139 or EP 2 645 133, a field emitting signal is coupled into a conductive underground structure by introducing a current from an AC current-source into soil by earth-spikes or other ground connection means, wherein the current follows along the conductive structure as path of least resistance through soil.
What all the mentioned detection systems have in common, is that the underground structures need to emit an electromagnetic field that is strong enough to be detectable above the surface, especially it has to be detectable non-ambiguously in respect of the always present noise-floor of various other electromagnetic fields from other sources. The electromagnetic fields emitted by different services reside in different ranges of frequency, dependent on the signals present on the service. Power-lines commonly provide currents with a fundamental frequency of 50 Hz or 60 Hz, dependent on the country, and therefore emit fields with the same fundamental frequency. But as e.g. described in EP 2 362 241, also harmonics of the above mentioned frequencies can be used for mains detection, in particular zero sequence harmonics.
Signals that are artificially applied to the structures (either by direct or by soil connection) are restricted in frequency by radio-communication-rules which are country-dependent and given e.g. to avoid interferences with radio communication services. A special example of frequencies allowed in the UK for a general geographic surveillance use, such as cable detection, are the frequencies of 8 kHz or 33 kHz, which are used by some CAT-equipment. For example, the VLF radio band range (=Very Low Frequency radio waves e.g. in the range of about 15 kHz to 60 kHz), especially the low wavelengths in the range of myriameter, are known to penetrate soil material quite well and can therefore be used for cable detection purpose.
The fields emitted by communication lines are another important detection target. For those services, no special single frequency can be expected but rather a range of frequencies has to be taken into account. Still, the emission of frequencies in those bands which are less strictly regulated will likely dominate.
For example as shown in WO 2011/104314, WO 2008/064851 or WO 2008/064852, the depth or distance to a buried service, which can be considered as a long current carrying conductor, can be determined according to the signal strength difference at two pickups located in a known spacing to each other.
A problem therein is that such a depth determination—which depth can be a rather critical value for excavation tasks—is quite sensitive to tolerances in the device's components and manufacturing process, in particular to the characteristics and arrangement of the detection coils.
Therefor, EP 1 843 177 describes a factory calibration rig, in which an individual fine tuning of each cable detection device can be determined in a factory or laboratory environment, in particular after device's fabrication or later on at a certification authority.
Once calibrated, the detection device is regularly exposed to quite harsh environmental conditions at worksites, heat concussions and vibrations in cars when transported, accidental dropping or knocking over, exposure to direct sunlight, snow, rain, water, dirt, etc. Therefore, the factory calibration data might be ill fitting in field operation. In particular, aging and temperature drifts of the electronics and a displacement of the coils in field-usage can have negative impact. Therefore, the guarantied accuracy levels of the depth values determined by such devices is in general relatively low, e.g. within some decimeters to meters.